Prior art geometry is shown in FIGS. 2, 3 and 4. These geometries use two pairs of mirrors which when moved apart on a common axis or on parallel axes, increase the optical path length by a factor of two times the change in the distance between the mirror assemblies. As shown, the prior art uses parallel paths to avoid tilting and translation of the beam. The prior art geometries have been realized either using mirror assemblies having two mirrors or a corner cube assembly. In either case the optical path length increases as the two assemblies are moved apart is two times the change in distance between the assemblies. In accordance with the present invention, the optical path length change can be double that of the prior art without adding optical elements.